This invention relates to illuminating instruments that reduce orientation nonuniformity and, more particularly, to an illuminating instrument that is optimal as an electronic flash unit for cameras or the like.
The assignee of this invention has proposed an electronic flash unit shown in FIG. 1 in Japanese Patent Application No. SHO 61-55742 (corres. to Japanese Patent Provisional Publication No. SHO 62-211627). The electronic flash unit comprises a semi-cylindrical reflector 13, and an elongated cylindrical flashing tube 17 arranged at a focal point of the reflector 13. A convex Fresnel lens 15 is arranged at an opening of the semi-cylindrical reflector 13. In the above electronic flash unit, the reflector 13 has a cross-sectional configuration which consists of an ellipse defined by the following equation: EQU x=y.sup.2 /R[1+{1-(1+k)y.sup.2 /R.sup.2 }.sup.1/2 ]
where k is a conical constant representative of the elliptic configuration of the reflector 13, and R is a radius of curvature of a reference spherical surface representative of an elliptic configuration pattern of the reflector 13. When it is supposed that an opening length of the reflector 13 in a cross-sectional plane perpendicular to an optical axis O of the reflector 13 and to an axis of the flashing tube 17 is D, an index of refraction n of the convex Fresnel lens 15 is equal to 1.4 through 2.0 and an illuminating angle .theta. is within a range of from .+-.15.degree. to 45.degree., a focal length f of the convex Fresnel lens 15, a depth Dp of the reflector 13, a spacing Fp between an apex of the reflector 13 and the focal position thereof, the constant k and the radius of curvature R are within ranges expressed by the following inequalities (1), (2), (3), (4) and (5): EQU 1.5D&lt;f&lt;3D (1) EQU -1&lt;k&lt;-0.4 (2) EQU D/10&lt;R&lt;D/2 (3) EQU 0.4D&lt;Dp&lt;1.2D (4) EQU D/20&lt;Fp&lt;D/3 (5)
With the arrangement described above, it is possible to form the reflector 13 so as to have the depth reduced, whereby the entire electronic flash unit can be miniaturized.
In the electronic flash unit constructed as above, however, the following problem has later been found. That is, as indicated by the broken line in a light distribution characteristic diagram shown in FIG. 2, the light quantity emitted in the vicinity of the optical axis O is reduced as compared with that in other directions, so that an uneven or nonuniform light distribution characteristic appears. According to an explanation of the inventors of this application, this is because "eclipse" occurs due to the flashing tube 17 per se. An influence of this phenomenon is remarkable if the front opening of the reflector 13 is small. The term "eclipse" does mean that light does not contribute to a condensing action due to any obstruction. The broken line in FIG. 1 indicates an example of light rays lacking due to "eclipse".